I am working on Arduino board for a while. One thing I do not get is why there is a chip called "USB to serial adapter" on Arduino board.
Isn't USB already serial. What does it mean USB to serial? Is it about voltage regulation only?
Yes, USB sends data serially. USB has one data signal carried differentially on two wires.
However, when people talk about "serial" communication in some contexts they don't mean the general serial as you are interpreting it, but a "serial port" also known as a "COM" port on PCs. Note that "COM" stands for "communication", so technically applies to all data links . "COM port" or "serial port" or sometimes just "serial" refer to RS-232 from context. Yes, this is all a bit sloppy, but it is common.
In your case "USB to serial adapter" almost certainly means something that presents like a normal device on the USB side but does UART-style communication on the other. There is yet another ambiguity in whether that UART-style communication is real RS-232 or using normal digital logic levels. The digital logic level signals are normally inverted from the true RS-232 signals. The microcontroller does "serial" via digital logic signals, but PC COM ports do real RS-232.
You're correct in that USB is serial at the physical level, however USB is a complicated protocol. Implementing it on the AVR micro would take up nearly all the program memory on the Arudino and leave no room for sketches!
The USB chip on the Arudiuno allows the board to talk to the Arudino IE without the overhead of implementing the USB protocol in firmware.